1. Field of the Invention
This invention relates to injection molding machines and, more particularly, to apparatus for ejecting parisons from the ejector portion of the molding apparatus.
2. Prior Art
Injection molding machines are widely used for the rapid and repeated molding of articles from thermoplastic materials such as polyethylene, polypropylyene, polyethylene terephthalate, and polyvinylchloride. One type of injection molding machine is used to form parisons or preforms which are to be placed in blow molding machines to form carbonated beverage containers. The typical parison has a hollow tubular body, a threaded neck defining an open end, and an annular flange adjacent the neck.
In such applications, polyethylene terephthalate is the preferred material. This type of injection molding machine typically includes an injector portion in which a molten thermoplastic material is injected into a mold cavity to form the parison, and an ejector portion which withdraws the solidified molded parison from the cavity and ejects it into a catcher or other suitable receptacle.
The injector portion of the machine typically includes a hopper for receiving beads of a selected thermoplastic material which communicates with a reciprocating helical screw mounted within a heated casing. The screw conveys the beads of thermoplastic material along the heated casing during which time the beads are heated and compressed to become a molten liquid mass. The casing necks down to an orifice in a fixed platen which communicates with a manifold, and reciprocation of the screw projects a charge of the material into the manifold. The manifold receives the molten plastic material from the orifice into a sprue and conveys it through a series of runners to a plurality of gates. Each gate communicates with a mold cavity and the plastic material flows into the mold cavity to form the parison.
The ejector portion of the injection molding machine typically includes a plurality of cores shaped to fit into the cavities and about which the molded parisons are formed. The cores are mounted on a base plate which is supported by a clamp plate attached to the movable platen so that the cores are capable of reciprocating movement into and out of the mold cavities. The movable platen is actuated by a primary double-acting hydraulic cylinder and is guided by a plurality of guide rods which extend from a fixed base in the ejector portion to the fixed platen on the injector portion which supports the mold cavity and manifold.
The rod of the primary cylinder has a recess within which is mounted a secondary or ejector cylinder. The ejector cylinder is a double-acting hydraulic cylinder that includes an actuator rod which extends through the movable platen into the frame to activate the ejector mechanism.
The ejector mechanism typically includes a stripper plate which reciprocates within the frame and consists of a plurality of slide bars extending transversely across the platen on either side of the cores. The slide bars have pairs of cams extending from their ends which engage cam slots formed in cam plates mounted on either side of the base plate and extending from the movable platen toward the fixed platen. The cam slots are parallel but diverge adjacent the fixed platen. The slide bars carry pairs of jaws or thread splits which enclose the cores. The reciprocating movement of the stripper plate by the ejector cylinder causes the jaws to alternately open and close about the core as the slide bars are brought away from and toward the cores and each other by the engagement of the cams with their cam slots.
During the injection molding process the movable platen is advanced until the cores are positioned within the mold cavities and the upper and lower members of the jaws are closed about the core forming a sealed chamber within which the parison is formed. Typically, the jaws have an interior surface part of which is shaped to form the threads on the neck of the parison. The reciprocating ram injects molten plastic material into the mold manifold and it travels through the runners to the individual mold cavities. The molten plastic material flows about the portions of the cores within the mold cavities and in between the upper and lower members of the jaws to form the parison. The mold cavity is cooled by a recirculating water system and after the injection process ceases the mold begins to cool.
After the molten plastic has cooled sufficiently so that the parison has solidified and become dimensionally stable, the movable platen is retracted away from the injector portion of the machine by the primary cylinder thereby removing the cores from the mold cavities. At this time, the parisons are carried on the ends of the cores and the threaded necks of the parisons are still in between the jaws.
At the start of the ejection step, the stripper plate is urged forward by the ejector cylinder and the jaws are spread apart by the action of the cams within the divergent cam slots. By the time the parison bodies have been removed from the ends of the cores, the jaws have been spread apart so that the threaded portion of the parisons no longer is tightly clasped by the jaws. The parisons then fall by gravity into an appropriate receptacle.
There are many disadvantages inherent in the type of injection molding machine previously described. Principally, such injection molding machines require ejector portions having many components requiring high dimensional tolerances which add to the cost of the apparatus. In addition, the design of the ejector portion prevents close spacing of the cores and hence the mold cavities. This reduces the number of parisons produced per injection cycle and thus limits the production capacity of the injection molding machine.
Accordingly, there is a need for a simplified parison ejector apparatus with lesser dimensional tolerance requirements and permiting a greater number of cores to be mounted on the movable platen so that a greater number of mold cavities can be employed, thereby increasing the production rate of a given injection molding machine. In addition, it is desirable to make such an apparatus capable of being retrofitted onto existing injection molding machines.